1. Field of the Invention
The present invention relates to a cordless window blind structure, and more particularly to a structure which allows greater extent of head rail cut-down on both distant ends and meanwhile increases the drop length of the window blind.
2. Description of Related Art
A conventional window blind structure is usually formed by a blind body, a driving assembly, and a pull cord. In operation, a user must draw the pull cord as a means to open or close the window blind to a desirable position. However, the small frictional surface of the pull cord tends to rub against the hands exerting force thereon, and thus hurting the area of hands contacting with it. Besides, in case of an excessive down-pulling force exerted onto the pull cord or a sudden release at a great speed, the blind body cannot be accurately positioned in a desirable position and the adjustment process must be repeated over again, which is quite inconvenient in operation and may hurt the user's hand.
In order to solve the problems of the conventional window blind operated by the pull cord, several techniques have already been disclosed in the prior art, for example, U.S. Pat. No. 4,760,622 and U.S. Pat. No. 5,477,904.
With reference to U.S. Pat. No. 4,760,622, the dual combination of the winding apparatus and counterbalance system function to save amount of force exerted by the user and meanwhile enabling the heavier windows to be raised in a counterbalanced way. The window has front and back panels installed at different rails, so that both the winding apparatus and counterbalance system must be installed in respective track, which complicates the whole structure and occupies quite a large space.
With reference to U.S. Pat. No. 5,477,904, it discloses a window curtain assembly having a spring tension mechanism. The extend-retract position of the window blind is operatively controlled by a handle without an exposed cord, so it is quite convenient for the user. However, as the inter-engagement of the spring tension mechanism and the whole window curtain assembly does not generate a friction force during practical operation thereby, the whole curtain body must be completely drawn out and then fastened to a buckle through a handle means. That is, the window blind is not capable of achieving a counterbalanced state at any particular position. Also, the spring tension mechanism must be correspondingly fixed to two utmost ends of a tubular frame (the head rail), so the structure of such window curtain assembly is not capable for a stock size window blind which may require cut-down at both distant ends in accordance with the customers' needs.
As an extendable-retractable spring is installed within the blind assembly as disclosed from above prior art, the associated components of the head rail cannot be disposed at the center portions of the head rail, and the relevant moving stroke of the components therealong the head rail can not be further reduced. However, this apparently is not a problem with the construction shown in U.S. Publication Patent No. 20070119547 due to the extension spring is replaced with a torsion spring.
The U.S. Publication Patent No. 20070119547 discloses a cordless window blind structure of which the user is not only allowed to operate the window blind in a counterbalanced way, but also the overall blind structure is further simplified. However, the associated head rail components (for example, a fixing base and a cord-winding control unit) must be respectively fixed on both utmost ends of the head rail, limiting the possibility for head rail cut-down at both distant ends.
In addition, when the movable seat is activated by a retrieving cord, this in turn will simultaneously generate a torque force causing the movable seat to decline or move sideway therealong the head rail, therefore, the movable seat must be made with a longer length to stay clear from this circumstance. However, once the movable seat is lengthened, this in turn will substantially shorten the distance between the movable seat and the cord-winding control unit, and further minify the spare space available within the head rail, therefore, the blind may not be suitable for cut-down needs. Accordingly, the extent of expansion of the blind body is corresponding to the lateral movement of the movable seat therealong the head rail. As described above, once the distance between the movable seat and the cord-winding control unit is shortened, this in turn may place limitations on the maximum stretch characteristics that the window blind can perform, in other words, the full length of the blind is restrained.
And the pulley block of the fixing base and the torsion spring assembly of the cord-winding control unit are respectively fixed at two utmost ends of the head rail thereby, the window blind is not suitable for cut-down performance.